Source : Webster's Revised Unabridged Dictionary (1913)

Eighteenth \Eight"eenth`\, n.
   1. The quotient of a unit divided by eighteen; one of
      eighteen equal parts or divisions.

   2. The eighth after the tenth.

Eighteenth \Eight"eenth`\, a. [From {Eighteen}.]
   1. Next in order after the seventeenth.

   2. Consisting of one of eighteen equal parts or divisions of
      a thing.

Source : WordNet®

eighteenth
     n : position 18 in a countable series of things

eighteenth
     adj : coming next after the seventeenth in position [syn: {18th}]